A variety of sensor systems have been developed for characterizing the occupant of a motor vehicle seat to determine whether to allow or suppress air bag deployment. A recurring requirement in this regard is the ability to reliably detect the presence of a rear-facing infant seat (RFIS), since nearly all vehicle manufacturers require that at least the front air bag be disabled in the case of a RFIS due to the proximity of the infant's head to the point of air bag deployment. In the past, the presence of a RFIS has been detected by measuring the occupant's weight distribution on the seat, by the state of the seat belt buckle and the seat belt tension, and/or by measuring the proximity or presence of the occupant relative to the point of deployment or the seat back or the passenger compartment ceiling. See, for example, Fu U.S. Pat. No. 6,024,378, Stanley U.S. Pat. No. 6,220,627, Patterson et al. U.S. Pat. No. 6,605,877 and Basir et al. U.S. Pat. No. 6,678,600. In some systems, the RFIS must be specially equipped with magnets or bar codes that are sensed by Hall sensors or scanners; see, for example, Meister et al. U.S. Pat. Nos. 5,678,854 and 5,570,903. What is needed is a way of simply and reliably detecting the presence of a RFIS without requiring the operator to utilize a specially equipped infant seat.